(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, and an MFP (Multi Function Peripherals), and more particularly, to an improvement of a heat fusing device for an image in the visible form.
(2) Description of the Related Art
For an MFP, media onto which are printed color images have been increasing. Taking print media of paper alone as an example, various types of paper that differ in thickness or the like are used. The types of paper are normally divided by the basic weight indicating a weight per a specific area (unit: g/m2), and for example, papers that fall within the ranges of 64 to 105, 106 to 163, 164 to 209, 210 to 256, and 257 to 300 are referred to as plain papers, thick papers 1, thick papers 2, thick papers 3, and thick papers 4, respectively. These basic weights depend on the density and the thickness of papers, and given that the density of papers is almost constant, then the basic weights are proportional to the thickness.
When the thickness of papers varies with the types of papers as described above, optimal print conditions to achieve a satisfactory print image vary as well. Generally, when the thickness of papers is increased, the fixing temperature of an image in the visible form drops abruptly, and when fixing is performed on the images at the same temperature, a wait-stop occurs due to poor fixing and a drop in temperature, which makes it impossible to achieve a satisfactory print image. Hence, in order to achieve a satisfactory print image, it becomes necessary to determine the type of papers used for printing.
In order to identify the types of papers, the user used to input the thickness and the basic weight on the screen displayed on the control panel of the MFP, and the fixing of the image in the visible form is controlled and a paper carrying rate is controlled according to the thickness and the basic weight of papers thus inputted.
The user, however, is rarely aware of the thickness of papers or the like except when he sets print papers in the print paper accommodating cassette. Hence, papers of wrong type are set often and the user fails to obtain a satisfactory print image, which results in a waste of print papers.
As is disclosed in JP-A-2004-126328, there is an image forming apparatus, in which plural temperature detecting elements are provided to a heating member to determine the type of a recording material according to a difference of temperatures detected by the temperature detecting elements, and the fixing temperature is set according to the determination result. However, because plural temperature detecting elements are required and the fixing temperature is changed in response to a difference of the temperatures thereof, there are problems that it takes a time to reach an optimal fixing temperature, the mechanism becomes complex, and so forth.